Treatment of spent sulfite liquor



June 6, 1961 R. P. WHITNEY ETAL 2,987,432

TREATMENT OF SPENT SULFITE LIQUOR Filed Aug. 6, 1957 7'0 sun-m? EMQWEESMELT WHTFK PZ/%I%IZTORS. 2 2 A y;

sou/770A! Patented June 6, 1961 2,987,432 TREATMENT OF SPENT SULFITELIQUOR Roy P. Whitney, Appleton, Wis., Shu-Tang Han, Roscoe,

Ill., and James F. Bakken and Richard B. Kesler, Appleton, Wis.,assignors to The Institute of Paper Chemistry, Appleton, Wis., acorporation of Wisconsin Filed Aug. 6, 1957, Ser. No. 676,533 7 Claims.(Cl. 162-36) The present invention relates generally to the preparationof wood pulp by a sulfite process employing a sodium base liquor. It ismore particularly related to improvements in the preparation of cookingliquor for the sulfite process from spent liquor.

Paper and paper products are manufactured from wood which has beenconverted into pulp. Wood pulp may be prepared by one of a number ofprocesses. Generally speaking, wood pulp is either prepared by amechanical process, or a chemical process. Chemical processes includethe acid sulfite, neutral sulfite, sulfate and soda processes. A morerecently employed process is the semichemical pulping process which hasachieved substantial importance because of the high yield of pulpobtainable therefrom. Since in the semichemical process, the wood issubjected to a milder treatment than in the chemical processes, it isreadily understood how higher yields result.

Wood treated in accordance with the semichemical pulping process may bedigested under substantially neutral conditions with a sodium compound.A well known process utilizes sodium sulfite in the cooking liquor inconnection with digestion of the pulp. This invention primarily relatesto the recovery of the spent liquor, i.e. the liquor remaining afterdigestion and removal of the pulp.

While various processes have been developed for recovering sodium base,spent sulfite liquor, they have not all been economical to practice norhave they provided a cooking liquor of high quality. Furthermore,previously known processes have required substantial capital investmentfor recovery of the liquor.

The pulp produced by the neutral sulfite semichemical pulping processespossesses strength and good bleachability characteristics. These andother characteristics, in view of the high yield, have establishedneutral sulfite semichemical pulp for use in the manufacture of paperand paperboard products. The semichemical pulp has been combined withother pulps in the manufacture of high grade paper, such as book paper.

In order to provide an overall process which is commerciallypracticable, various attempts have been made to recover the spent liquorby concentration and burning to provide a smelt. The smelt is dissolvedto furnish an aqueous solution which is then treated with sulfur dioxideto form sodium sulfite. However, this technique as usually practiced inthe past has resulted in the formation of excessive amounts ofthiosulfates and other polythionic compounds which are not effective indigestion and result in a dead load in the system.

Attempts have also been made to oxidize smelt solutions in order toprepare a cooking liquor. These attempts have not resulted in acommercially practicable process for the paper industry.

Similarly, carbonation processes for recovering spent liquor have notbeen adopted.

A suitable recovery method for spent liquor from a sodium base, sulfitepulping process has been discovered and it is set forth in United Statespatent application, Serial No. 382,678, filed September 28, 1953, nowUnited States Patent No. 2,802,791, issued August 3, 1957, assigned tothe assignee of this invention. This process involves the treatment ofsmelt recovered from spent sulfite liquor. The smelt is dissolved andtreated under particular conditions to minimize the contact betweenhydrogen sulfide and sulfur dioxide.

We have now discovered a new method whereby a cooking liquor of highquality for a sodium base, sulfite pulping process can be prepared fromspent liquor in a simple and economical manner without substantialformation of thiosulfates and other polythionic compounds so that a highquality liquor results. In addition, by this method, hydrogen sulfidefrom a sulfitation operation may be recovered in a concentrated form.The recovered hydrogen sulfide is converted to sulfur dioxide and thento sodium bisulfite, the sulfitation agent of the method. The method maybe carried out in a continuous or batch operation and may be utilized inthe preparation of acid sulfite liquor or neutral sulfite liquor.

In the method, sodium bisulfite, in aqueous solution, is reacted with anaqueous smelt solution under particular conditions to provide a highquality cooking liquor.

A main object of the present invention is to provide an improved methodfor handling the spent liquor from a sodium base, sulfite pulpingprocess.

Another object of this invention is to provide a high quality cookingliquor from spent liquor from a sodium base, sulfite pulping process bydirect sulfitation with sodium bisulfite.

It is also an object of this invention to provide a cooking liquor,substantially free from sodium thiosulfate or other polythioniccompounds, for the sodium base, sulfite semichemical pulping process orfor the sodium base acid sulfite pulping process, through the use ofsodium bisulfite.

Further objects of this invention will be obvious from a study of theaccompanying drawing and following disclosure. In the drawing:

FIGURE 1 is a schematic drawing of a portion of a recovery system whichmay be employed in the recovery of spent sulfite liquor.

FIGURE 2 is a schematic drawing of a conversion tower included in thesystem shown in FIGURE 1.

In accordance with the invention, spent liquor from a sodium base,sulfite pulping process is concentrated to form about 55 to about 70percent solids content in a suitable manner. The concentrated liquor isthen burned under reducing conditions so as to limit the removal ofsulfur as much as possible. In this connection, a major portion of thesulfur should be retained in the resulting smelt. The smelt formed byburning primarily comprises sodium sulfide and sodium carbonate. Thesmelt is then dissolved to form an aqueous solution of the sodiumsulfide and sodium carbonate.

The sodium sulfide and sodium carbonate are then converted to thedesired sodium sulfide. This conversion is effected by directsulfitation of the aqueous smelt solution with sodium bisulfite. Thereaction occurring during the sulfitation may be represented by thefollowing equations:

Thus, not only is the sodium sulfide converted to sodium sulfite, but soalso is sodium carbonate.

Since an equilibrium exists between sodium bisulfite and sulfurdioxide-sodium sulfite and since hydrogen sulfide reduces sulfurdioxide, a condition exists which results in the formation of sodiumthiosulfate. This may be represented by the following equations:

Accordingly, it appears that the preseuce of hydrogen Sincesulfurdioxide is the common sulfitation agent 1 in the conventional commercialprocess, formation of sodium thiosulfate during direct sulfitation hasbeen generally accepted.

From the foregoing, however, thefo rmation ofthiosulfate depends uponthe initial reaction between the hydrogen sulfide and the sulfurdioxide, with the resulting formation of elemental sulfur. Consequently,when the contact bctween'these'two agents is minimized, the

formation of thiosulfate is accordingly minimized; It

has been found that sulfur dioxide'does not react with sodium' sulfide,and will not react with'sodium hydrosulfide to'form thiosulfate.

The smelt from the furnace should have'a sulfidity of between aboutpercent and about 70' percent. The term sulfidity refers to the molarratio of sodium sulfide in the smelt to the sum of'the sodium sulfideand the sodium carbonate in the smelt; Of course,- this ratio ismultiplied by 100 to provide the percent.

'-In addition, the reduction of the smelt must be greater 3 than 85percent, the percent reduction being the molar ratio of sodium sulfideto sodium sulfide plussodium sulfate, times 100.

'It is within the skill of the art to provide burning conditions inarecovery furnace so as to give a smelt having the desired degree ofreduction and-the desired sulfidity.

As has been previously indicated; this invention is based upon theestablishment of certain conditions for minimizing the contact'betweenthe hydrogen sulfide and sulfur dioxide, when a smelt solution fromspent sulfide 40 liquor is reacted with a sodium bisulfi'desolution soas to provide aliquor of high quality. For purposes of com-'mercialoperation, the gross quality of the liquor should beinexcess' of85 percent and the net quality should exceed '80'percent. At' lowerliquor qualities," the w coveryprocess is not commercially'feasible andthe digestion of the pulp is not as satisfactorily carried out;

I Th termfgross'quality refers'to the molar ratio of the amount-ofsodium sulfiteto-the-amount of sodium sulfite plus-theamount of sodiumthiosulfate-formediu the reaction. The term net quality" refers to theamountofsodiurn sulfite recovered -from-the sodium sulfidein the smeltto the amount of such sodium sulfite plus -'theamount of sodiumthiosulfate.- e

The smelt-solution-should be at a boiling temperatureand the contactbetween thesolution and sodium bisulfiteshould be under conditions whichestablish "alarge surface area and immediate stripping of anyformed-hydrogensulfide gas. In this connection, in a batch operationytheboiling rate of the smeltsolutiofishould exceed 40 grams of vapor perminute'per square foot; of surface; area to 6 obtain 100percent-conversion'bf-the sodium sulfide in the smelt solution. While'lower'boiling rates can be-tolerated at-lower degrees of conversion toprovide'the desired net quality, i.e. quality in excess of 80 percent, aboiling rate of less than about '15 grams of vapor per minute persquarefoot of surface area'is not satisfactory.

Because of the inherent difficulties in carryingout the batch operation,the preferred method for practicing this invention involvesmixing-ofthesmelt solution, at high temperature, for a short period and thenspraying the solution into an open chamber. In this connection,'the

e time of contact betwecn the'solutions should'not exceed five seconds,and the time is preferably less-than second. The solution; is; sprayed;as indicated, and stripping steam is flowed through the spray at a ratein excess of 0 per minute per gallon of liquor sprayed.

25 pound moles of steam per pound mole of sodium sulfide in the smeltsolution, i.e. the total amount of stripping steam employed should be atleast 25 pound moles of steam per pound mole of sodium sulfide in thesmelt solution. For the most satisfactory operation, the stripping steamshould exceed 40 pound moles of steam per pound mole of sodium sulfide..In order that the steam can act effectively, the spray should establisha surface area on the drops in. excess of 100 square feet The area ofthe droplets and the amount of stripping steam should be adjusted sothat the hydrogen sulfide in the resulting liquor is retained at a ratioof less than .14 mole per mole of sodium sulfide introduced in spray.form and preferably less than .12 mole per mole of sodium sulfideintroduced. If the retained hydrogen sulfide is present in a ratio ofless than .12 mole per'mole of sodium sulfide in: the liquor sprayed;and otherconditions are maintained, the net quality will exceed. about:percent.

The concentration of sodium sulfide in the smelt solution should. be nomore than about 2 gram moles per liter, themaximum concentration ofsodium sulfide being controlled: by the solubility limit of sodiumsulfite in the resulting liquor;

Inorderto provider satisfactory continuous operation, themole ratio ofthe amount of sodium bisulfite to the amount. of sodium sulfite'plussodium bisulfite in the solutionv whichis mixed with the. smelt solutionshould not be greater than about .90. The use of higher ratios 0 resultsin a lower liquor quality. Of course, too low a ratioresultsininefficient operation. 7 At a ratio above .7 excessive sulfur lossesoccur in condensation of the exhausted gases.

.In order to effect most satisfactory treatment of the 5 smelt:solution, the pH of the aqueous smelt solution Compound: Lbs. NagsogNaacog; NaHCO 51 N32830:; Na- SO -2. 14

Total 470 accordance with this invention, isthen concentrated.

After concentration, theliquor is a combustible concentrate and willordinarily contain-aboutbipercent solids. Itisthempassedto-andburnediniafurnacqsuch as a Babcock and Wilcox furnace, to providetheidesiredreductionand a smelt having the required sulfidity.

, The'smelt from the-furnace -isdissolved ,inwater in tank 5 t o providethe dilutegaqueoussolution: which :is to be sulfited. This SOllltlOIllSpassed;toclarifier IIlfllB clarifier, the solution is separated from'anyiundis'solved solida; It=is-1then pumped to a storagejtanktlh Ananalysis {of a: typical smelt solution; or: this. in-

vention, leaving the dissolving mask 5.. isggivembelow. The analysis onthe basis of one ton of pulp.

Compound: Lbs. Na s 1'47 NaHS 30 NaOH 22 Ma oo 114 Na SO 14 Total 327The smelt solution is then pumped to a conversion tower 11 in which thesolution is converted to the desired treating liquor. Upon suitableconversion, the liquor is then stored in a tank 13 for use in thedigester.

Conversion tower 11 may be of the type shown in FIGURE 2 for continuousoperation. The regeneration system of FIGURE 2 may be used in thepreparation of either neutral sulfite or acid sulfite liquor.

As shown in FIGURE 1, the smelt solution, or green liquor, is introducedinto a pump 15 and is mixed with steam to heat it, whereupon it isintroduced into the conversion tower 11. Sodium bisulfite liquor (aspreviously pointed out, this liquor includes sodium sulfite), which hasbeen heated by steam, is also pumped from a pump 17 into the conversiontower 11. The gases which leave the conversion tower are introduced intoa cyclone separator 19, which returns any liquid to the tower 11. Thegases leave the cyclone 19 and enter a condenser 20 and the remaininggas, referred to as sour gas, is passed through a fan 21 to a sulfurburner (not shown). A portion of the liquor which has been recovered ascooking liquor leaves the regeneration system 11 through the pump 23,and another portion of the liquor is mixed with the green liquor andintroduced into the pump 15. Thus, some of the liquor is recycled.

The conversion tower :11 basically comprises three sections, they beinga spray chamber 31, a packed section 33 and a sump 35. The lowermostsection is the sump 35. Superimposed thereon is the packed section 33,and above the packed section is the spray chamber 31. The packed section33 may be filled with any of several commercially available packingmaterials, such as partition rings, spiral tiles, Raschig rings, Berlsaddles, wire-mesh packings, or other suitable types of conventionalabsorption tower packing material.

The smelt solution enters conversion tower 11 through pipe 37. Thesodium bisulfite solution enters the conversion tower '11 through pipe39. The pipes 37 and 39 each connect to nozzles 41 which spray the mixedliquor and bisulfite solution into the spray chamber. The sprayed liquorthen passes through the packed section 33 to the sump 35. Y

The tower is operated at the boiling point of the combined spray, thatis, at least about 212 F. to 214 F. under atmospheric pressureconditions. Preferably, suflicient extra heat is continuously added inthe liquor and thereby to the spray to provide an increased boilingrate.

During passage of the boiling spray down through the packing, itcountercurrently contacts steam which has entered the sump 35 of theconversion tower 11. Steam entering through line 40 passes up throughthe packed section 33 of the tower 11. The steam is at a temperaturewhich is a function of the pressure conditions in the tower and thesteam aids in stripping hydrogen sulfide formed during the reactionbetween the sodium'bisulfite and the sodium sulfide in the spray. Thesteam has the additional eifect of aiding in maintaining the downfiowingspray solution at the desired temperature.

Hydrogen sulfide and other efiiuent gasses pass from the tower throughstack 43 after flowing around the baffle 45. Battle 45 is provided so asto substantially eliminate exiting of inflowing bisulfite solution sprayand smelt solution spray before their passage down through the packedsection to the sump 35.

When the liquor reaches the sump 35, it passes from the tower 11 throughpipes 47 and 49, pipe 47 conducting 6 the liquor to the storage tank 13and pipe 49 conducting the liquor to the pump 15 for recycling.

It will be understood that the tower shown in FIGURE 2 may be operatedon a batch basis by filling the sump 35 melt solution.

EXAMPLE I An eight inch stainless-steel conversion tower of the typeshown in FIGURE 2 was operated on a batch basis in accordance with theconditions set forth in Table I below.

Table I BATCH BISULFITE PROCESS [Spray method] Run. I 11 Smelt Solution:

olume, litels 7. 0 5.0

N82320:, g. mo1es 0.035 0 040 Nflgsog, g. moles..- 0. 042 0.040

N aIS, g. moles 0. 588 2. 323

NazOO g. moles 0.721 2 315 Bisulfite solution:

Volume, liters 12. 2 6. 1

N a180,, g. moles 0. 037 2.075

NaHS0 g. molt- 2. 365 9. 250 Reaction solution:

Reacting time, min 120 80 Volume, liters 35. 7 15. 2

N M810 g. moles 0.071 0. 639

Na1SO3, g. moles 2. 538 10. 60

NaHOO;, g. mo1es 0.350 Recycling rate, gal/min 1 1 Nags conversion, molepercent 100 Na CO; conversion mole percent. 100 92. 5 Gross quality,mole percent 98. 6 93. 5 Net quality, mole percent 96. 5 87. 8

1 Estimated volume based on sodium balance.

sary to achieve minimal boiling of the combined solution.

EXAMPLE II In a commercial unit, the smelt includes the followingmaterial in the specified amounts:

Moles per ton Sodium sulfide 2.90

Sodium sulfate .35

Sodium carbonate 1.78

About 488 pounds of this smelt is introduced into the dissolving tank 5at about 1800 F. In clarification, about one percent of the chemicals islost. Water is added, about 4940 pounds being added to the smelt at F.The clarified green-liquor is discharged at about 194 F. and conductedto the conversion tower 11 which, in this case, is operated underatmospheric pressure. The flow rate is 46 gallons per minute.

The spray nozzle pressure in the conversion tower is 50 psi. and thesmelt solution is heated to a temperature of 250 F. The cooking liquoris recycled from the sump 35 so that the recycled liquor has a ratio tothe entering green liquor of 3 to 1, by volume.

Bisulfite solution is introduced at a ratio of bisulfite to sodiumsulfite of 7 to 3, on a molar basis. The bisulfite liquor is at atemperature of 205 F. at the pulp 17. The liquor has the followingcomposition:

Steam is introduced through pipe 40 at the rate of Exit liquor BisulGreen 'fite Liquor Liquor for for for Recy- Bisul- Cookcling tite ingNaisleonversionjmole percent. N810 O; reacted, moles/ton. NazGconversion, mole percent; Liquor quality, mole percent NmSO; formed,moles/ton.. NfizSgOz formed, moles/ton" Gross quality, moles percentNmSO; formed trom Nags,

moles/ton Net Quality, mole percent-.-

.The overall data on the operation. of .the conversion.

tower 11 for a commercial operation is set forth below:

Lb./ton P l Green liquor 5, 790 Bisulfite liquor (205 F. 13, 240

N S0 485. 935. 193. 99. 11,530. 7:3 by mole. 1,485 gaL/ton or 103gaL/min. 17, 680

39. Water 15,830. Recycling-green liquor ratio--. 3:1 by volume. Flow1,985 gaL/tou or r 138 gaL/mm 7 Steam at atmospheric pressure M 2, 600

V Total input 39, 310 Exit liquor (215 F.) 36,830

NBQSO:

Spray chamber dimeusions.. ..14 it. D. 4 it.

7 Packed section 4.5 it. D. x 10 it.

6ft.'D.x5ft. Sour gas exit. l0-in. Packing 2-111. glazed porcelainRasehig rings;

Steam rate, packed section. 280 lb./ (hr.) (sq. ft). Liquor rate, spraychamber. 1,000 lb./(hr.) (sq. ft.). Liquor rate, .packedsection--..10,000. lb./(hr.). (sq. in). Spray rate 350,000 sq. ttJmln.

varied within the specified limits, in relation to one another, toprovidethe desired results of thisinvent'ion. While the ep'e'rsfiaeenaifians can "he vanes; conditions appear to be' at aisstannalimportance. In

1 this connectionfthe reads-embarrass the bisulfitev solu- $51 9... acient sodium bisulfitc to react with substantially all of merit of. thehigh boiling conditions, 10'

tion and the -snie'lt selfitien'snsnddeenrnnaernign boiling rateconditions Thetiiiie of es-hue: between bisulfite solution and the smeltsolution should be minimized which, bf'dufs, "is"pi'oniotedby theestablish- To aid in minimizing this contact time, the spray area, i.e.the droplets discharged trom the nozzles, is preferably increased asmuch as possible and stripping steam is employed over this area in aparticular amount. The concentration of sodium sulfide should bemaintained at a relatively low level in the smelt solution and theconcentration of sodium ion should be also maintained at a comparativelylow level. If the proper smelt has been provided, a liquor having highnet quality and high gross quality can be provided. Of course, the pH ofthe solutions should be above 7.0 and the pH of the initial smeltsolution is desirably above 11.0. 7

As before indicated, the present invention may be practiced by utilizingspent liquors from either the sodiurn base, neutralsulfite pulpingprocess orthe sodium base acid sulfi-te pulping process. In the ventthat the batch or continuous method of recovery is utilized with spentliquor from the acid sulfite pulping process, the aqueous liquorrecovered from the conversion tower 11 may be subjectedto a furthertreatment with sulfur dioxide to increase its acidity. This may be donein another conversion tower by furt-her sulfitation with sulfur dioxidegas. a

In the foregoing discussion, continuous and batch processes have beendescribed for recovering spent liquors from neutral sulfite and acidsulfite pulping processes in an easy and economical manner. Theprocesses of this invention are independent of other processes.

It will be evident that various modifications in the procedural stepsand/or equipment for carrying them out canbe made inthe processesdescribed without departing firom the scope of the present invention.

The various features of this invention whichare believed to be new areset forth in the following claims.

We claim:

1. In "a sodium base, sulfite pulping process, the improvement whichcomprises the steps of concentrating the spent liquor from the'pulpingprocess, burning the concentrated liquor to provide a smelt having asulfidity of between about 30 percent and about percent, the reductionof said smelt being in excess of 8.0 percent, dissolving said smclt toprovide an aqueous solution and boiling said solution having a sulfiditybetween about 30percent and about 70 percent at a rate in excess of .15grams of vapor per minute per square foot of surface area duringaddition of sodium bisulfite solution to said aqueous solution in anamount sufiicient to provide suificient sodiurn bisulfite to react withsubstantially all of the sodium sulfide in ,said aqueous solution,whereby direct-sulfitation occurs, and rapidly removing'the hydrogensulfide from the resulting boilingsolution, whereby formation of sodiumthiosulfate and associated substances is substantially avoided. f

2. In a sodium base, sulfite pulpingv process, the. improvement whichcomprises the steps of concentrating the spent liquor from the pulpingprocess, burning the concentrated liquor to provide a smelt having asulfidity of between about 45 percent and about 7 0 percent, thereduction of said smelt being in excess of percent, dissolving .saidsineltto provide aqueous solution and boiling saidsolution having asulfidityof between about 45 percent and about 70 percent ata rate inexcess of 40 grams of vapor per minute per square foot of; surface a tes. s iti ois s i 'bi u fi u i n tgsaid olution in a amount suflicientto providesuflithe sodium sulfide in said aqueous solution, wherebydirect sulfitation occurs, and rapidly removing the hydro gen sulfidefrom the resulting boiling solution, whereby formation of sodiumthiosulfate and associated substances is substantially avoided.

3. in a sodium base, sulfite pulping process, the improvement whichcomprises the steps of concentrating the spent liquor from the pulpingprocess, burning the concentrated liquor to provide a smelt having asulfidity of between about 30 percent and about 70 percent, thereduction of said smelt being in excess of 80 percent, dissolving saidsmelt to provide an aqueous solution and mixing said aqueous solutionhaving a sulfidity of be tween about 30 percent and about 70 percentwith a sodium bisulfite solution in an amount sufiicient to providesufiicient sodium bisulfite to react with substantially all of thesodium sulfide in said aqueous solution for a total contact time of lessthan 5 seconds at a boiling temperature, whereby direct sulfitationoccurs, spraying the mixed solution, and passing stripping gas throughthe sprayed mixed solution, the amount of said gas exceeding 25 poundmoles per pound mole of sodium sulfide in said aqueous solution, wherebyformation of sodium thiosulfate and associated substances issubstantially avoided.

4. In a sodium base, sulfite pulping process, the improvement whichcomprises the steps of concentrating the spent liquor from the pulpingprocess, burning the concentrated liquor to provide a smelt having asulfidity of between about 30 percent and about 70 percent, thereduction of said smelt being in excess of 85 percent, dissolving saidsmelt to provide an aqueous solution having a sulfidity of between about30 percent and about 7 percent, the concentration of sodium siulfide insaid smelt solution being less than about 2.0 gram moles per liter andmixing said aqueous solution with a sodium bisulfite solution in anamount sufiicient to provide sufficient sodium bisulfite to react withsubstantially all of the sodium sulfide in said aqueous solution for atotal contact time of less than .1 second at a boiling temperature,whereby direct sulfitation occurs, spraying the mixed solution, andpassing stripping steam through the sprayed solution, the amount ofsteam exceeding 40 pound moles per pound mole of sodium sulfide in saidaqueous solution, whereby formation of sodium thiosulfate and associatedsubstances is substantially avoided.

5. In a sodium base, sulfite pulping process, the improvement whichcomprises the steps of concentrating the spent liquor from the pulpingprocess, burning the concentrated liquor to provide a smelt having asulfidity of between about 45 percent and about 70 percent, thereduction of said smelt being in excess of 85 percent, dissolving saidsmelt to provide an aqueous solution having a sulfidity of between about45 percent and about 70 percent and mixing said aqueous solution with asodium bisulfite solution in an amount suflicient to provide suflicientsodium bisulfite to react with substantially all of the sodium sulfidein said aqueous solution for a total contact time of less than secondsat a boiling temperature, whereby direct sulfitation occurs, sprayingthe mixed solution, said spray establishing a surface area at a rate inexcess of 100 square feet per minute per gallon of solution sprayed, andpassing stripping steam through the sprayed solution, whereby formationof sodium thiosulfate and associated substances is substantiallyavoided.

6. In a sodium base, sulfite pulping process, the improvement whichcomprises the steps of concentrating the spent liquor from the pulpingprocess, burning the con centrated liquor to provide a smelt having asulfidity of between about 45 percent and about percent, the reductionof said smelt being in excess of percent, dissolving said smelt toprovide an aqueous solution having a sulfidity of between about 45percent and about 70 per cent, the concentration of sodium sulfide insaid smelt solution being less than about 2.0 gram moles per liter andmixing said aqueous solution with a sodium bisulfite solution in anamount sufficient to provide sufiicient sodium bisulfite to react withsubstantially all of the sodium sulfide in said aqueous solution for atotal contact time of less than 5 seconds at a boiling temperature,whereby direct sulfitation occurs, said bisulfite solution includingsodium sulfite, the molar ratio of sodium bisulfite to so dium siulfiiteplus sodium bisulfite being less than .9, spraying the mixed solution,and passing stripping steam through the sprayed solution, the amount ofsteam exceeding 40 pound moles per pound mole of sodium sulfide in saidaqueous solution, whereby formation of sodium thiosulfate and associatedsubstances is substantially avoided.

In a sodium base, sulfite pulping process, the improvement whichcomprises the steps of concentrating the spent liquor from the pulpingprocess, burning the concentrated liquor to provide a smelt having asulfidity of between about 45 percent and about 70 percent, thereduction of said smelt being in excess of 85 percent, dissolving saidsmelt to provide an aqueous solution having a sulfidity of between about45 percent and about 70 percent and having a pH in excess of 11.0 andmixing said aqueous solution with a sodium bisulfite solution in anamount suflicient to provide sufficient sodium bisulfite to react withsubstantially all of the sodium sulfide in said aqueous solution for atotal contact time of less than .1 second at a boiling temperature,whereby direct sulfitation occurs, spraying the mixed solution, andpassing stripping steam through the sprayed solution, the amount ofsteam exceeding 40 pound moles per pound mole of sodium sulfide in saidaqueous solution, whereby formation of sodium thiosulfate and associatedsubstances is substantially avoided.

References Cited in the file of this patent UNITED STATES PATENTS2,800,388 Ahlborg July 23, 1957 2,802,791 Whitney Aug. 13, 19572,864,669 Ahlborg Dec. 16, 1958 FOREIGN PATENTS 534,769 Canada Dec. 25,1956 OTHER REFERENCES Sodium Base Sulfite Recovery as Related to theSemichemical Process; article by T. T. Collins et al.; Southern Pulp andPaper Manufacturer (Journal); January 1956, Atlanta, Ga., 12 pp.; pp.10-12 relied on.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No; 2 987432 June 6 1961 Roy Po Whitney et ale It is hereby certified that errorappears in the above numbered patent requiring correction and that thesaid Letters Patent should read as corrected below.

Column 2. line 41 for "form" read from ====g column 4 line 74 for maskread tank ==g column 6 Table I line 11 thereof for 'Reaction" readReacted column 8 line 25 for "vent read event column 9 line 33 for"'siulfide" read sulfide column 10 line 2O for "siulfiite" read sulfiteSigned and sealed this 24th day of October 19610 SEAL) Lttest:

ERNEST W. SWIDER Ittesting Officer DAVID L. LADD Commissioner of PatentsUSCOMM-DC

1. IN A SODIUM BASE, SULFITE PULPING PROCESS, THE IMPROVEMENT WHICH COMPRISES THE STEPS OF CONCENTRATING THE SPENT LIQUOR FROM THE PULPING PROCESS, BURNING THE CONCENTRATED LIQUOR TO PROVIDE A SMELT HAVING A SULFIDITY OF BETWEEN ABOUT 30 PERCENT AND ABOUT 70 PERCENT, THE REDUCTION OF SAID SMELT BEING IN EXCESS OF 80 PERCENT, DISSOLVING SAID SMELT TO PROVIDE AN AQUEOUS SOLUTION AND BOILING SAID SOLUTION HAVING A SULFIDITY BETWEEN ABOUT 30 PERCENT AND ABOUT 70 PERCENT AT A RATE IN EXCESS OF 15 GRAMS OF VAPOR PER MINUTE PER SQUARE FOOT OF SURFACE AREA DURING ADDITION OF SODIUM BISULFITE SOLUTION TO SAID AQUEOUS SOLUTION IN AN AMOUNT SUFFICIENT TO PROVIDE SUFFICIENT SODIUM BISULFITE TO REACT WITH SUBSTANTIALLY ALL OF THE SODIUM SULFIDE IN SAID AQUEOUS SOLUTION, WHEREBY DIRECT SULFITATION OCCURS, AND RAPIDLY REMOVING THE HYDROGEN SULFIDE FROM THE RESULTING BOILING SOLUTION, WHEREBY FORMATION OF SODIUM THIOSULFATE AND ASSOCIATED SUBSTANCES IS SUBSTANTIALLY AVOIDED. 